1. Field of the Invention
The present invention relates to a servo signal processing apparatus for carrying out a focus servo by detecting a focusing state according a laser beam returned from an optical disk or an optical-magnetic disk, and an optical disk apparatus.
2. Description of the Prior Art
In a conventional optical disk apparatus for recording and/or reproducing a data signal on/from a disk-shaped recording medium such as an optical disk, a focus servo is carried out by detecting a focus error signal.
That is, the conventional optical disk apparatus applies a laser beam to an optical disk and carries out a focus servo according to a focus error signal detected from a laser beam returned from the optical disk.
Detection of a focus error signal is carried out by a focus detection device, and a focus servo is carried out by a servo signal processing apparatus provided in the optical disk apparatus.
The aforementioned focus detection device is, for example, disclosed in Japanese Patent Laid-Open Sho 60-217535. This focus detection device emits a laser beam to an optical disk and receives the laser beam reflected from a signal recording plane of the optical disk for detecting a focus error signal.
That is, as shown in FIG. 1, in the aforementioned focus detection device, a laser beam is emitted from a light source 101 and goes through a beam splitter 102 and a collimator lens 103 and is focused by an objective lens 104 onto a signal recording plane of an optical disk 105.
The laser beam reflected from the signal recording plane of the optical disk 105 goes through the objective lens 104 and the collimator lens 103 and is reflected further by the beam splitter 102 to enter a beam splitter 106.
The beam is branched in the beam splitter 106 so as to be received by a first photo-detector 107 and a second photo-detector 108.
The first photo-detector 107 and the second photo-detector 108 are arranged so as to be in an identical distance before and after a conjugate point (as a reference) of a laser focusing point of the laser beam reflected from the signal recording plane of the optical disk 105 in a just focus state.
As shown in FIG. 2, the first photo-detector 107 and the second photo-detector 108 respectively consist of three photo-detection blocks (107a, 107b, 107c, and 108a, 10b, 108c).
Here, a focus error signal is detected by comparing a total of an electric signal from the photo-detection blocks 107a and 107c of the first photo-detector 107 and an electric signal from the photo-detection block 108b of the second photo-detector 8 with a total of an electric signal from the photo-detection block 107b of the first photo-detector 107 and an electric signal from the photo-detection blocks 108a and 108c of the second photo-detector. The focus servo is carried out so as to make this focus error signal zero.
However, the aforementioned focus detection device requires two beam splitters 102 and 106 and has such a configuration that the light source 101, the beam splitters 102 and 106, and the photo-detectors 107 and 108 are arranged separately from one another. Consequently, the optical disk apparatus requires a complicated assembly in a considerably large size with a high cost.
Furthermore, an laser beam branched in the beam splitter 106 is received by the photo-detector 107 and the photo-detector 108 for detecting a focus error signal. Consequently, if any change is caused in the reflection characteristic of a reflection film formed on the beam splitter 106, the intensity of beam detected by the photo-detectors 107 and 108 is greatly affected.
Therefore, if any irregularities are contained during a manufacturing of the beam splitter 106, irregularities are also contained in the intensity of the beam received by the photo-detector 107 and in the intensity of the beam received by the photo-detector 108. This means that, for example, just focus cannot be obtained even if the focus error signal is zero.
That is, even if focus servo functions so as to make the focus error signal zero, defocusing is caused and a data signal recorded or reproduced to/from the optical disk is deteriorated.